Recent developments in machine technology have realized higher output and rotation rate of engines and motors, and as a result, demand has arisen for a high-performance lube oil which endures severe use conditions. In addition, in order to cope with energy and environmental problems, such a lube oil is required to have fuel consumption reduction effects and energy saving effects as essential performance characteristics. Recently, the lube oil must further have a long-life (long-drain) performance from the viewpoint of resource savings.
Under such circumstances, in the future, the lube oil is required to have as low a viscosity as possible for reducing viscosity resistance which would otherwise cause power loss; sufficient heat resistance; and durability under long-term use conditions.
Generally, lube oil is an organic material predominantly composed of hydrocarbon. Therefore, when viscosity of the lube oil is reduced, vapor pressure of the oil inevitably increases, resulting in loss of the lube oil via evaporation and increasing flammability. Particularly when the lube oil is employed as, for example, hydraulic fluid in facilities where high-temperature objects are handled; e.g., machines in an iron mill, the lube oil must have non-flammability, from the viewpoint of fire prevention. In precision motors employed in information-related apparatuses (e.g., hard disk apparatuses) which have been developed in recent years, a lube oil having resistance to evaporation and diffusion is demanded in order to minimize adverse effect on other precision apparatuses placed therearound.
In order to solve such problems, hitherto, fatty acid esters, silicone oils, and fluorocarbon-based oils such as perfluoro-polyether have been proposed as lube oils which have low viscosity and high heat resistance despite low vapor pressure. However, these proposed materials have drawbacks. Specifically, fatty acid esters have poor water resistance, due to the ester structure, which is highly susceptible to hydrolysis. Although silicone oils and fluorocarbon-based oils have excellent heat resistance and water resistance, these oils exhibit poor lubricity as compared with conventional hydrocarbon-based lube oils. Thus, there has never been provided a lube oil totally meeting strict demands which are to be required more and more in the future.
Meanwhile, in recent years, it has been reported that, among organic ionic liquids each being formed of a cation and an anion, a class of ethylimidazolium salts having a variety of anion moieties exhibit excellent thermal stability and high ionic conductivity and assume liquid stable in air (see, for example, Patent Document 1). Thereafter, interest in these ionic liquids has grown rapidly, and extensive studies on the liquids have been carried out. A variety of applications such as electrolyte in solar cells (see, for example, Non-Patent Document 1) and solvents for extraction/separation and reaction have been envisaged on the basis of various characteristics of the ionic liquids including thermal stability (volatilization resistance and non-inflammability), high ion density (high ionic conductivity), large heat capacity, and low viscosity. However, there have never been reported cases in which the aforementioned organic ionic liquids are employed as lube base oils.
In ionic liquid, molecules thereof are bonded via ionic bonds, which are stronger than intramolecular forces as found in molecular liquid. Therefore, ionic liquid is resistant to volatilization, is non-flammable, and is stable against heat and oxidation. In addition, since the ionic liquid exhibits low volatility despite having low viscosity, and has excellent heat resistance, it may be the only lube oil that would meet strict demands required in the future. However, physical properties of ionic liquid greatly depend upon ionic bonds between molecules. Thus, differing from the case of molecular liquid such as liquid hydrocarbons, physical properties of ionic liquid are difficult to predict from the molecular structure thereof, and properties such as viscosity, viscosity index, and pour point cannot readily be controlled through modification of the molecular structure. In other words, design and synthesis of an ionic liquid compound having target physical properties are difficult, which is problematic.
In addition, ionic liquid per se is a salt formed of a cation and an anion. Therefore, an ionic liquid formed of a certain cation-anion combination is dissolved in water in an arbitrary amount (see, for example, Non-Patent Document 2). Although such an ionic liquid does not decompose or cause corrosion under anhydrous conditions, the ionic liquid absorbs water under hydrous conditions and may decompose or cause corrosion. Among ionic liquids having excellent heat resistance, species having an ion (e.g., an imidazolinium ion) are oxidative or highly susceptible to reduction decomposition (see, for example, Non-Patent Document 3), and those having another ion (e.g., BF4− or Cl−) have toxicity and impose a heavy environmental load. Thus, in order to obtain a lube oil meeting a strict demand, rigorous selection of constituent ions is preferred.
Furthermore, ionic liquid, which is formed of a positively charged cation and a negatively charged anion, also has electrical characteristics; e.g., alignment in accordance with an electric field and formation of an electric double-layer on an electrode surface. By virtue of the aforementioned electrical characteristics, when an electric field is applied to a lubrication site where ionic liquid is present, electrical characteristics will be developed, possibly varying tribological characteristics to a certain degree.
There have conventionally been disclosed methods for regulating friction including application of an electric field to a system employing a lube oil. For example, some methods employ a dispersion-type electrical viscous fluid in which solid particles are dispersed in a liquid medium (see, for example, Patent Documents 2 and 3), and others employ a homogeneous electrical viscous fluid which is formed of a liquid crystal homogeneous solvent (see, for example, Patent Document 4). All these methods regulate tribological conditions through modification of physical properties of electrical viscous fluid (i.e., increasing viscosity). Therefore, when friction conditions such as shear rate and load become too severe to overcome, the effect commensurate with increase in viscosity often fails to be attained.
[Patent Document 1]    Japanese Patent Application Laid-Open (kokai) No. 2003-31270
[Patent Document 2]    Japanese Patent Application Laid-Open (kokai) No. Heisei 5(1993)-25488
[Patent Document 3]    Japanese Patent Application Laid-Open (kokai) No. 2000-1694
[Patent Document 4]    Japanese Patent Application Laid-Open (kokai) No. 2000-130687
[Non-Patent Document 1]    J. Chem. Soc., Chem. Commun., 965(1992)
[Non-Patent Document 2]    “Ionic Liquids: The Front and Future of Material Development,” CMC Publishing CO., LTD.
[Non-Patent Document 3]    “M. Ui, Curr. Top. Electrochem., 7, 49(2000)